A well known optical fiber cable design comprises a core which comprises at least one coated optical fiber, a longitudinally extending core tube in which the core is disposed and a sheath system which includes an outer jacket longitudinally disposed about the tube. The optical fiber cable often includes, additionally, strength members disposed between the core tube and an outer surface of the outer jacket.
Optical fiber cables and associated apparatus such as a cable closure, for example, are typically designed and manufactured to avoid problems which may be caused by a variety of anticipated environmental conditions to which the cable and associated apparatus will be exposed. One such condition is the exposure of portions of the cable and the cable closure to water. Potential problems associated with this condition are the entry of water into internal portions of the cable as well as the entry of water into the cable closure in which the cable has been terminated, spliced or branched.
The entry of water into a cable closure is an undesirable condition because water generally has a detrimental effect on the enclosed optical fiber cable, the optical fiber and internal portions of the cable closure. For example, water vapor may attack surface flaws of the external surface of the glass optical fiber and cause stress corrosion. In the presence of water and stress in the optical fiber, a surface flaw in the optical fiber will tend to grow in size, causing a reduction in the surface area over which the stress in the fiber is distributed. This may result in lower tensile stress fiber breaks. Another detrimental effect resulting from the presence of water in a closure may be the influence of the water's freezing. The freezing of water may subject the fiber and internal portions of the cable and closure to damaging mechanical loads.
Sealing an optical fiber cable closure to prevent the ingress of water depends, generally, on effecting three different seals. A first seal necessary to properly seal a closure is a seal formed between a portion of an outer surface of an outer jacket of the cable and a surface of the closure or a surface that is independently sealable with the cable closure. This may be accomplished by various means including gland packing fittings wherein a pliable gland is caused to be compressed into sealing engagement with a portion of the outer surface of the cable jacket and a portion of a surface of the gland packing fitting. This first seal is effective to prevent the ingress of water between a portion of the outer surface of the outer jacket and the cable closure.
A second cable closure seal necessary to prevent the ingress of water into a cable closure is a seal associated with a closure cover. The cover is caused to seal with, for example, a closure bulkhead portion through which cables extend into the closure by means that include lip seals, "O" rings, gaskets or gel-like sealant materials.
A third seal necessary to prevent the ingress of water into a cable closure is a seal disposed about an end portion of the cable which is disposed within the cable closure. This seal needs to be effective to prevent water that enters interior portions of the cable at a point outside the cable closure from traveling along the length of the cable between adjacent cable sheath components. A cable end portion that includes this third seal is said to be water blocked. It is this third type of seal which is addressed by this invention.
Water may have an occasion to enter the core tube through a cut in the cable which extends through the outer jacket and the core tube. Because the core tube communicates with an interior portion of the cable closure in which an open end of the core tube is disposed, water traveling along an interior portion of the core tube could enter the cable closure.
An annular gap between adjacent layers of the internal cable structure may also act as a conduit through which water may travel to enter the cable closure. Water may enter a portion of the internal cable structure through an unintended cut or puncture in the outer jacket of the cable and travel longitudinally within the cable.
Water blocking compounds such as, for example, a jelly-like material are often disposed within the core tube and are effective to prevent substantially the entry of water therein, as well as to prevent water that does in fact enter the core tube from traversing longitudinally along the interior portion thereof. Means such as water blocking tapes, as disclosed in C. J. Arroyo U.S. Pat. No. 4,867,526, are effective to arrest the migration of water within portions of the cable structure.
The efficacy of such water blocking means is measured as an ability of it to prevent an absolute flow of water over a specified length of the cable at a specified liquid head pressure for a given period of time. Cable industry standards for flow of fluids within a cable have been promulgated and have set forth standard test conditions, equipment, procedures and failure criteria. One such standard is promulgated by the Electronic Industries Association A 455-82, and is entitled, Fluid Penetration Test For Filled Fiber Optic Cable. Standard test conditions set forth in A 455-82 include an applied fluid head of 1.0.+-.0.1 meter of water, factory atmospheric temperature and pressure, and a test sample length of cable of 1.0.+-.0.1 meter. Pursuant to the test procedure, water is caused to be introduced into an end of the test cable, the pressure of which is maintained at 1 meter of static water head for a period of one hour. Any evidence of water leaking from another end of the cable is sufficient to fail the test cable.
What is important to realize is that in a case where a cable is terminated inside a cable closure, water can enter the closure by way of a path within the cable even though the cable otherwise passes the fluid penetration tests described above. Water entering through a cut or puncture in the outer jacket of the cable may be able to enter the closure in which the cable is terminated if the cut or puncture is located at a distance from the closure less than the test length of the above stated test, that is, 1.0 meter. Therefore, means must be provided to prevent the ingress of water into a cable closure via internal cable paths other than an internal portion of the core tube of an optical fiber cable.
One method of preventing the ingress of water into a closure utilizes a potting compound. Here, a closure may include two sections defined by two bulkheads with cable sealing grommets through which cables extend and a closure cover which is tubular and which has a closed end and an open end. A cable end portion is extended through a first bulkhead and cable sealing grommet. Portions of the cable are removed, that is, all of the cable sheath components including an outer jacket, strength members and other intermediate members disposed about the core tube, so that only an end portion of the core tube extends from the cable end portion. The core tube is caused to be extended through a second bulkhead spaced from the first bulkhead and a sealing grommet. The cable portion is positioned such that the end portion of the cable comprising the outer jacket and other sheath components and an adjacent, relatively short length of the core tube are caused to be disposed within a first chamber of the closure defined by the first and second bulkheads and a portion of an interior surface of the closure cover. The core tube is caused to communicate with a second chamber which is defined by the second bulkhead and an end portion of the closure cover and which is opposite the first chamber.
In a closure in which potting compounds are used to effect water blocking of the cable sheath, splices are typically located in the second chamber. The water blocking of the cable end portion against the flow of water through portions of the cable sheath components into the cable closure occurs in the first chamber. The closure cover is fastened securely and the first chamber is caused to receive a liquid potting compound. The compound is allowed to cure, thus forming a solid potting compound. The solid, cured potting compound is effective to prevent water from entering the cable closure through portions of the cable sheath components.
The just-described cable water blocking arrangement is most practical when frequent access to the interior of the cable closure is not anticipated. Reentry into the closure would most often occur if a new cable were being spliced or connected to another cable within an in-use closure. To effect the introduction of a new cable into the closure, at least a portion of the cured potting material within the closure would have to be removed. The first chamber of the closure is re-potted after a portion of the new cable is introduced to reestablish the closure water blocking capability. The removal of cured potting material and the installation of new potting material is an expensive, time consuming and labor intensive process.
What is needed and what does not appear to be provided in the prior art is an arrangement for causing a cable portion such as an end portion which is located within a closure to be water blocked in a way that is inexpensive, does not require the use of potting materials and allows reentry of the closure without destroying the water blocking seal of other, previously installed cables.